Show user-defined member buttons in toolbar.

For this whole thing to work the list of memberTypes is passed to the
Docs app, which then uses the list to create toolbar buttons.
Use shorter member type names in toolbar buttons.

Add additional :toolbar_title setting to @member_type config.
Use it to assign shorter toolbar titles to the member types that
otherwise have a bit too long titles.
Define member subsections in Tag#member_type config.

No more special casing for config options subsections - the Tag::Cfg
just defines different rules for splitting members into subsections.

Instance/Static subsections are now only defined for properties and
methods.  Events and CSS members will be listed without subsectioning.